This is already mainstream. Facebook even pulls the code directly from your SMS if you are logging into their Android app. It's just that it's a second factor

This is already mainstream. Facebook even pulls the code directly from your SMS if you are logging into their Android app. It's just that it's a second factor to the password. Your phone can be stolen as well (although such theft is harder to scale) - so keeping the knowledge-test around is important. I agree though, if you don't care about security, OTPs are potentially much more convenient than traditional passwords.